As energy-saving and environmental issues have become increasingly prominent, lithium ion (Li-ion) batteries, having advantages of high specific energy and being environmentally friendly, have been widely used in various fields such as electric automobiles, aerospace, and ships due to their relatively large capacity power supply. With the development of li-ion battery technology, an energy density of the li-ion battery becomes higher and higher and a quantity of battery units in a battery pack also may increase. After using the battery pack for a particular duration of time, asymmetry existing among the batteries or cells in the battery pack may cause one or more of the batteries overcharging or over-discharging, and may subsequently lower the performance of the battery pack as a whole. This may eventually affect the service life of the battery pack in a negative manner. For the battery pack, a battery management system for managing and monitoring a working state of the battery pack is desirable.
In practice, state of charge (SOC) is an important reference parameter of the working state of the li-ion battery pack, and may be used to indicate a remainder energy of the li-ion battery pack. Accurate SOC estimation of the li-ion battery pack utilized on automobiles can not only tell drivers of correct estimated mileage of the automobiles, but also may ensure improved charging/discharging of the li-ion battery pack, which is a guarantee of safe utility of the li-ion battery pack. When the automobile is running, large currents may cause the battery pack to over-discharge and may subsequently destroy the battery pack. Therefore, real time collection of voltage, temperature, and charging/discharging current of each battery is important for accurate SOC estimation of the battery so as to prolong the life of the battery pack and increase performance of the automobile.
The SOC estimation may use attribute parameters of the battery such as voltage, current, resistance, temperature of the battery. The attribute parameters of the battery generally may change as the battery may age and with respect to other uncertain factors, such as random road conditions the automobile may experience.
In present, the most popular method for SOC estimation of battery is ampere hour method, which is also a relatively accurate method on SOC estimation. The ampere hour method employs real time current integral to calculate ampere hour, and then revises temperature, self-discharging data and ageing parameters that can affect the SOC estimation, and eventually obtains a relatively accurate SOC value by use of a revision function and said parameters. However, the above-mentioned method is still far away from practical situations, due to there being many other factors that may practically affect SOC estimation of the battery and, it is difficult to achieve the revision function in practice. Therefore, to date the SOC value estimated by employing ampere hour method is far away from the real SOC value of the battery. Other existing methods for SOC estimation include constant current/voltage method, open circuit voltage method, specific density method, and so on. These methods each have more or less defects that may lead inaccurate SOC value.